1. Field of the Invention
This invention relates to a portable power unit, such as a portable generator, and more particularly to a portable power unit having the function of preventing an excessive load current from flowing from part of a plurality of power units connected with each other for parallel operation due to variation in the output voltage characteristics between the power units.
2. Prior Art
When a plurality of power units, e.g. portable generators, connected in parallel with each other are in operation, if the output voltages from the generators are not synchronous, there occurs a difference between the output voltages. This causes electric current to flow from one generator into another generator, which can result in such an excessive amount of electric current flowing in the one generator as may destroy component parts thereof. Therefore, it is required to synchronize the output voltages from the generators.
To this end, even in the case of parallel operation of identically-constructed portable generators, it is required to provide wiring for transmitting signals for informing each other of operating conditions thereof. Further, as disclosed e.g. in Japanese Patent Publication (Kokoku) No. 56-20782, it is required to employ an automatic synchronizing device, and means for producing an operating point at which the phases coincide with each other in order to cause the automatic synchronizing device to operate promptly and reliably. Alternatively, as disclosed in Japanese Provisional Utility Model Publication (Kokai) No. 62-145440, a special adaptor has to be used for parallel operation of two portable generators, with one of them serving as a master generator while the other as a slave generator.
As an improvement in the prior art, the present assignee has proposed, in U.S. Ser. No. 07/824,178, now U.S. Pat. No. 5,258,700 a portable power unit which dispenses with the use of special means such as an automatic synchronizing device and a special adaptor for carrying out parallel operation of a plurality of portable generators.
This type of portable power unit is usually provided with a protective circuit for cutting off the supply of electric current to a load device when the power unit is in an excessively loaded state to thereby protect the output circuit, etc. off the power unit. However, when a plurality of power units of this type are connected together for parallel operation, as shown in FIG. 1, a greatest amount of load current flows from one power unit (one having an output characteristic S1 in the example shown in FIG. 1) having the highest output voltage level within the tolerance of the rated output voltage of the power units, so that the one power unit shares the greatest part of load, due to variation in the rated output voltage between the power units. The variation arises due to variations in manufacturing tolerances and temperature characteristics of circuit component parts between the power units. Although the variations are slight, they are very difficult to avoid. As a result, the operation of the protective circuit of the one power unit from which flows the greatest amount of load current determines the timing of protective operation for the whole power units in parallel operation even if outputs from the other power unit(s) is (or are) much lower than its (or their) capacities. Thus, the total output power from the whole portable power units in parallel operation is undesirably limited by the aforementioned operation of the protective circuit, which prevents parallel operation of the power units from fully achieving its purpose, i.e. increase in the output power. More specifically, in the example shown in FIG. 1, when the power unit having the output characteristic S1 outputs its maximum current IA, another power unit having an output characteristic S2 outputs only a current IB. As a result, the maximum total output power PM from the power units in parallel operation is limited as shown below: EQU PM=VM (IA+IB).
In this connection, .increment.V in FIG. 1 designates a variation in the output voltage between the power unit.